In recent years, attention has been paid to that a nanostructural crystal exhibits peculiar optical-characteristics in a semiconductor including ultrafine particles and porous silicon that are typified by silicon (Si) and germanium (Ge). By the term nanostructural crystal as mentioned herein is meant crystalline particles having a particle diameter of several nm, approximately. Thus a phosphor having a nanostructural crystal is generally designated as nanocrystal phosphor or nano-phosphor.
The phosphor is employed for vacuum ultraviolet excitation luminescent element such as plasma display panel (PDP), and for instance, practical application is performed by using Zn2SiO4:Mn2+ as a green phosphor, Ba Mg Al10 O17:Eu2+ as a blue phosphor and (Y, Gd) BO3:Eu2+ as a red phosphor.
In recent years, a trend towards high precision, fineness and luminance has steadily been advanced in an image display unit which displays color images such as PDP using cathode-ray-tube and field emission display (FED) which is expected as next-generation thin display, whereby higher luminescence efficiency is required of a phosphor. In order to respond thereto, there have been developed phosphors having better characteristics. However, a phosphor with sufficiently satisfactory characteristics has not yet been obtained.
Moreover in recent years, a demand for a nanosized phosphor is rapidly increased. The reason for this is that consideration is given to the applications of a nano-phosphor having high transparency and light transmittance in the fields of information security, medical machinery and equipment, building, interior and the like other than an image display unit such as PDP and FED.
Importance should be attached to the improvement of a green phosphor, since it occupies 70% of luminance on a white screen. As a typical example of materials for green phosphors, a Zn2SiO4:Mn2+ based phosphor is cited.
As processes for producing Zn2SiO4:Mn2+ based phosphor, there are known (1) solid phase process, (2) sol-gel process, (3) hydrothermal synthetic process, (4) synthetic process using supercritical water and (5) synthetic process using supercritical ethanol.
(1) In the case of solid phase process, crystalline Zn2SiO4:Mn2+ is synthesized by firing a mixture of an oxide and/or a carbonate at a high temperature of around 1000° C., wherein Zn2SiO4 is formed at 900° C., and at 1000° C. or higher, almost single phase Zn2SiO4 is obtained, but depending upon the handling before and after the firing, there are caused such problems as deterioration in luminescence characteristics, lattice strain and lattice defect. Further there is raised a problem of ZnO sublimation making it impossible to maintain a stoichiometric ratio according to the charged chemical composition. In addition, it is impossible for the solid phase process to nanosize a phosphor, since the particle diameter of fired particles is much larger than that in a liquid phase process and further, large particles are agglomerated by firing at a high temperature. When the fired particles are pulverized by means of a ball mill or the like, there is also created a problem of deterioration in luminescent intensity due to lowered crystallinity.
(2) In the case of sol-gel process (refer to C. Cannas, M. Casu, A. Lai, G. Piccaluga, “XRD, TEM and 29Si MAS NMR study of Sol-Gel ZnO—SiO2 nanocomposites” J. Mater. Chem. 9, 1765-1769 (1999), it is difficult for the process to synthesize without growing nanoparticles, since firing at 800° C. or higher is necessary.
The above-mentioned problems due to synthesis at a high temperature are common to a variety of composite oxides typified by Zn2SiO4. In order to solve such problems, investigations have recently been made on (3) a hydrothermal synthesis process by the use of an autoclave [refer to T. S. Ahmadi, M. Haase, H. Weller, “Low temperature synthesis of pure and Mn-doped willemite phosphor (Zn2SiO4:Mn) in aqueous medium”, Material Research Bulltin, 35, 11 1869-1879 (2000)]. According to the hydrothermal synthesis process, it is possible to obtain Zn2SiO4:Mn having high crystallinity is obtainable by repeating dissolution and deposition at around 250° C. under high pressure without requiring a firing step. Nevertheless, it is impossible for the process in question to obtain a product having a uniform particle size.
(4) A proposal has been made on a synthesis process using supercritical water, which however, is problematic in that the process cannot be conducted with an ordinary autoclave because of severe working conditions over critical point of water (critical temperature of 374.1° C., critical pressure of 22.04 MPa).
Under such circumstances it has been found by the present inventors that nano-phosphor Zn2SiO4:Mn2+ can be synthesized by the use of (5) nitric acid and supercritical ethanol (critical temperature of 243.0° C., critical pressure of 6.14 MPa), and the above-mentioned finding was already published [refer to the published data on “Low temperature synthesis and optical properties of Zn2SiO4:Mn2+ phosphor in a supercritical ethanol solvent” by Takuro Miki, Tetsuhiko Isobe in the 69th Congress of Japan Electrochemistry Association, April, 2002].
However the above-mentioned process involves the problem in that since the nitric acid acts as an oxidizing agent and carbonizes the ethanol, allowing the resultant carbon to remain in the matrix of the phosphor, it is colored brown and thus the luminescent intensity is deteriorated. Moreover, when further miniaturization of color dot is required as a display material, it is impossible to cope therewith by the foregoing process using nitric acid along with ethanol.
Such being the case, it is required to assure a nano-phosphor having higher luminescence efficiency and at the same time, establish a process for producing a nano-phosphor having higher luminescence efficiency which process is capable of controlling the configuration of the substance.